1. Field of the Invention
This invention relates to an ultrasonic Doppler flow imaging and/or measuring system.
2. Description of the Background Art
This invention pertains to ultrasound imaging techniques, and finds particular use in medical diagnostic applications. It is known that a medical ultrasound imaging system can be used to display and analyze anatomical structures within a patient's body. The ultrasound imaging system transmits sound waves of very high frequency, typically 2 to 10 MHz, into the patient's body and processes echoes reflected from tissues and materials within the patient's body. A number of different types of displays are provided by ultrasound imaging systems but probably the most popular display is a two-dimensional (2D) image of selected cross-sections of the body. In an echo mode of operation, all echoes from a selected cross-section are processed and displayed. Use of the echo mode of operation enables a sonographer to detect a number of anatomical defects. Further, the size of such defects can be more or less precisely determined. The performance of the echo mode of operation is determined by the size of a resolution cell and, as is well known, the size of a resolution cell can be decreased by utilizing dynamic focusing and dynamic (matched) filtering.
In some clinical applications, anatomical defects can be relatively small, and echoes produced by such small anatomical defects are overshadowed by larger echoes from surrounding tissue. However, such small anatomical defects may be seen by displaying changes in blood flow velocity. As is well known, Doppler measurements can be used to determine the velocity of a moving object and a display of Doppler shifts caused by blood flow enables small anatomical defects to be detected more easily. This mode of operation wherein Doppler shifts caused by blood flow are displayed is known in the art as Color Flow. For example, U.S. Pat. No. 4,800,891 issued to Kim, and assigned to the same assignee as the present invention, describes the color flow process and how Doppler information relating to blood flow velocity can be gathered from large selected cross-sections of an anatomical structure under study. A color flow processor is used to develop estimates of three spectral moments of a flow signal, e.g., its power, velocity, and variance. These estimates are then used to cause the ultrasound system to display a 2D color flow image during a color flow mode of operation.
It is difficult to acquire sufficient ultrasound data to develop an accurate, high resolution, blood flow image at a high rate. Thus, in order to obtain more precise Doppler information about blood flow velocity from a small cross-sectional area, as is well known, the spectral Doppler mode of operation is used, such as described, for example, in the article entitled "Extraction of Blood Flow Information Using Doppler-Shifted Ultrasound", by Halberg and Thiele, published in the Hewlett-Packard Journal, pp. 35-40, June 1986. In the spectral Doppler mode of operation it is possible to devote more time to a selected small area. The results of the spectral Doppler mode of operation are conventionally displayed by means of a frequency spectrum and an audio signal.
One of the more problematic areas in diagnostic Doppler processing is effective removal of unwanted signals from tissue reflections which, due to their large amplitude, can overshadow and thereby mask the desired blood flow signals. Removal of signals arising from tissue movement is conventionally accomplished with a high pass filter (HPF). The design of this filter incorporates certain assumptions about the maximum frequency of tissue (or clutter) signals and the necessary corresponding cut-off frequency of this HPF. The filter is then designed according to this cut-off frequency. However, in practice, the spectrum of clutter signals is not stationary, due to cardio-pulmonary motion in the human body and also the intentional and non-intentional movement of the ultrasound probe by the operator of the diagnostic equipment.
Similar problems arise in radar applications of similar techniques (i.e. processing techniques for echoes of electromagnetic signals), and many different techniques for the removal of unwanted signals (generally referred to as "clutter") have been used. The reference entitled "Radar Handbook", by M. I. Skolnik (McGraw-Hill Book Co., New York, 2nd Ed. 1990) describes several of these techniques. One such technique described in this reference is known as the TACAR system for airborne radar systems.
A method similar to the TACAR system, and adapted for application to ultrasound systems, is described in U.S. Pat. No. 4,961,427, issued to Namekawa. In this approach, as well as in the TACAR system, compensation for clutter movement is carried out on the received signal during its downconversion to baseband, using an RF signal mixer.
In another approach, described in U.S. Pat. No. 5,170,792, issued to Sturgill et al., compensation for clutter movement is performed on the received signal after its frequency downconversion to baseband signal components. Since the baseband signal is a complex signal (i.e., having In-phase (I) and Quadrature (Q) components), tissue movement (clutter) compensation is carried out during a complex mixing operation using a tissue velocity signal as the complex reference signal. In each of these previously mentioned approaches, as well as all other approaches known to the inventors herein, although special circuits may be used for estimation of tissue (clutter) velocities, only the change of mean tissue frequency is compensated for by the reference signal. However, in an ultrasound system, tissue movements are also accompanied by changes in other spectral moments of the tissue signal, e.g., its power and spectral width. Changes of these parameters are not considered by the foregoing techniques, and therefore the effectiveness of prior art tissue/clutter rejections based only on compensation of tissue mean velocity is relatively limited, especially in diagnostic ultrasound applications.